The present invention relates to a running state control system for a motor vehicle.
Some modern motor vehicles are equipped with a speed control apparatus which automatically controls a vehicle speed to a setting established by a driver.
Such a speed control apparatus is disclosed, for example, in JP-A 59-58134. When driving a motor vehicle having a speed control apparatus of this type, a driver accelerates the vehicle up to a speed at which he desires to run it in a constant manner, then, he presses a vehicle speed setting button to set the speed. Under these conditions, the vehicle undergoes a speed control to adjust the vehicle speed to a setting by regulating an opening degree of a throttle value or a carburetor.
This speed control function is useful for the driver when driving on a superhighway at a constant speed, because continuous depression of an accelerator pedal is not needed. This function is canceled when the driver depresses a clutch pedal or a brake pedal. The vehicle is accelerated over the setting when the accelerator pedal is depressed, whereas the vehicle is decelerated by an engine brake, then runs again at the set speed in a constant manner when the accelerator pedal is released.
On the other hand, some modern motor vehicles are equipped with a traction control apparatus to prevent a skid of driving wheels.
Such driving force control apparatus is disclosed, for example, in JP-A 62-91326. With a motor vehicle having a traction control apparatus of this type, when the vehicle undergoes a sudden acceleration during starting period on a road with extremely low friction coefficient such as a frozen road, or when the vehicle makes a skid during a slow acceleration upon high speed run, an advanced reduction in traction of the vehicle is possible in accordance with the magnitude of a skid ratio and/or a variation thereof. Thus, the high running stability is assured in any running pattern.
To decrease the traction of the vehicle, a throttle valve called "second throttle valve" is closed. This second throttle valve is connected in series with a mechanical throttle valve which is opened by a known accelerator pedal. The second throttle value is completely opened and is closed by a closing operation signal from the traction control apparatus. The traction is determined by that one of the mechanical throttle valve and the second throttle valve which is more closed.
If a running state control system for a motor vehicle is of a combination of the known speed control apparatus and traction control apparatus, there arise the following problems:
1--Referring to FIG. 3, when the traction control apparatus closes the second throttle valve on a road with low friction coefficient, the vehicle speed is decreased as indicated by a portion B of a characteristic curve 1. If the speed control apparatus is in operation then, the mechanical throttle valve will be opened by a command of the speed control apparatus as indicated by a portion A.sub.1 of a characteristic curve 3.
Under such conditions, when the vehicle comes to a road with friction coefficient (point a in FIG. 3), the traction control apparatus opens the second throttle valve in an instant (point b in FIG. 3). On the other hand, the speed control apparatus, which is relatively slow in response, gradually opens the mechanical throttle valve as indicated by a portion A.sub.2 of the characteristic curve 3.
This results in a temporary generation of a traction greater than the required one to keep a vehicle speed C set in the speed control apparatus. Thus, when the vehicle comes again to the road with high friction coefficient, the vehicle undergoes a quick acceleration as indicated by a portion B.sub.1 of the characteristic curve 1, resulting in a run at a speed over the set speed C as indicated by a portion B.sub.2 of the characteristic curve 1.
This is due to presence of the opening operation signal to the mechanical throttle valve outputted by the speed control apparatus during the driving force control.
2--When the speed control apparatus is mounted to the motor vehicle having an automatic transmission, one of the functions of this apparatus is to recover the vehicle speed by increasing the traction of the vehicle based on an automatic down-shift of the gear position when the vehicle comes to a rise in the road, and that the vehicle speed becomes less than the setting by a predetermined value or more.
When the traction control apparatus is in operation on the road with low friction coefficient, however, an increase in traction by the down-shift of the gear position is not effective.
3--If the operation of the speed control apparatus in to be discontinued, the mechanical throttle valve which is opened then will be closed with the least possible delay.
However, if this closing operation of the mechanical throttle valve takes place on the road with low friction coefficient where the traction control apparatus begins to function, the running stability of the vehicle may be decreased due to quick reduction in traction of the vehicle.